The present invention relates generally to seismic surveying and specifically to a method for resolving the effects of formation anisotropy in shear wave reflection seismic data.
Exploration geophysicists have generally considered the earth's subsurface formations to be locally homogeneous and isotropic. See for example Devaney and Oristaglio, Geophysics, v. 51, No. 2, p. 419-423 (February, 1986). However, seismologists investigating earth parameters in the study of earthquakes have begun to consider the effects of anisotropic earth media on the propagation of seismic waves in the following articles:
S. Crampin and R. McGonigle, "The Variation of Delays in Stress-Induced Anisotropic Polarization Anomalies," Geophys. J. R. astr. Soc. (1981), v. 64, pp. 115-131.
C. Keith and S. Crampin, "Seismic Body Waves in Anisotropic Media: Reflection and Refraction at a Plane Interface," Geophys. J. R. astr. Soc. (1977), v. 49, pp. 181-208.
C. Keith and S. Crampin, "Seismic Body Waves in Anisotropic Media: Synthetic Seismograms," Geophys. J. R. astr. Soc. (1977), v. 49, pp. 225-243.
D. Booth and S. Crampin, "The Anisotropic Reflectivity Technique: Anomalous Reflected Arrivals from an Anisotropic Upper Mantle," Geophys. J. R. astr. Soc. (1983), v. 72, pp. 767-782.
D. Booth and S. Crampin, "The Anisotropic Reflectivity Technique: Theory," Geophys. J. R. astr. Soc. (1983), v. 72, pp. 755-766.
S. Crampin, "Seismic Anisotropy--a Sumary," J. Geophys., v. 43 (1977), pp. 499-501.
S. Crampin, R. McGonigle and D. Bamford, "Estimating Crack Parameters from Observations of P-wave Velocity Anisotropy," Geophysics, v. 45, No. 3 (1980), pp. 345-360.
S. Crampin, "Seismic-Wave Propagation Through a Cracked Solid: Polarization as a Possible Dilatancy Diagnostic," Geophys. J. R. astr. Soc. v. 53, No. 3 (1978), pp. 467-496.
S. Crampin, "A Review of the Effects of Anisotropic Layering on the Propagation of Seismic Waves," Geophys. J. R. astr. Soc. v. 49 (1977), pp. 9-27.
All of the above have focused on theoretical or observational investigations regarding the propagation of seismic waves in the earth or mathematical models or laboratory media having an anisotropic character with applications to the deep interior of the earth.
Recent acquisitions of exploration shear wave reflection seismic data have often yielded confusing and distorted seismic data which cannot be interpreted. Applicant has found that a significant cause of such confused and distorted shear wave reflection seismic data can be traced to the generally accepted assumptions regarding locally homogeneous and isotropic earth formations. Consequently, Applicant has developed a novel method of shear wave seismic exploration to ameliorate the deleterious effects of formation anisotropy. Moreover, Applicant's novel method of shear wave seismic exploration can be employed to detect and resolve azimuthal variations in the earth's subsurface formations resulting from preferentially oriented fractures and intrinsic anisotropy.